Integrating Time

If you’ve been following the posts on Integration Nation, you’re no doubt familiar with a variety of reasons why a system designer might choose to use (or not use) an integrated analog solution for a given project.

Reducing space and cost are common reasons for integrating. If you have space constraints, integration may help reduce board real estate. If you’re designing a price-sensitive consumer product, integration may allow you to reduce costs compared to a discrete implementation. But in many cases, the main reason for integration is not to reduce size or cost, although either may be a secondary benefit. Instead, the best reason for taking advantage of analog integration is often that when it’s done right, it can help reduce the time you invest in your design.

So, the right chip not only integrates circuit blocks, it also integrates time.

Here’s a simple example: a portion of an AFE signal path consisting of an amplifier, an ADC, and a voltage reference. You can clearly build this function using individual ICs. Selecting each of the blocks -- ensuring they meet your performance requirements, stay within your board area limits, and hit your cost target -- certainly isn’t rocket science, but it can be a time-consuming slog, first through a bunch of parametric selection tables, then through a bunch of data sheets to find the right combination of components.

A “perfect” solution can be elusive. Maybe a critical specification for an otherwise-ideal component has just a typical data sheet value rather than a maximum limit. (So do you keep looking, or try to find a way to change your architecture so that the specification is no longer critical?) Or perhaps what looked like the right op amp won’t settle fast enough when driving the input stage of the ADC you’ve selected. (Do you start searching for a different amplifier, or a different ADC?) Or maybe the voltage reference with the right combination of output voltage, supply current, initial accuracy, and drift exceeds your cost target. (Can you find a way to relax the requirements, or do you start looking for cheaper versions of the other components to keep the overall cost in line?)

Those sorts of dead ends and decision points continue to eat up your time.

Now, this was a simple example involving just three components (ADC, reference, and amplifier) that are easy to find integrated together. When more circuit functions are integrated, the potential development-time benefits increase.

The integrated design -- again, assuming it’s done right -- can eliminate a big chunk of the component search process. Even better, all of the pieces will work together (they’d better -- it would be a major design fail if they didn’t), which should further reduce the hours you put into the project. And a really good integrated design may also take care of some design details that you would have otherwise needed to work out.

A single data sheet can tell you whether the circuit has a high probability of fitting into your design or not. If it’s a good fit, you’ve just saved a lot of time -- a precious commodity in any design environment.

Kerry - the point you were making righjt at the end - cost - yep. I went thru those problems at Intersil when selling to customers. We would have component X that the customer wanted and really liked the specs for - mostly. But the wanted to know if the device (e.g.) work down to -50C instead of merely -40C. And we knew it would, just because we knew how the silicon performed in general. But we never tested it at -50C since only 1 customer in 100k needed that spec - so we couldn't guarentee (by virtue of testing during manufacturing) that spec. Or we could if wwe wanted to triple the price.

I designed a 12 lead ECG amplifier. Though it is called a 12 lead, 10 signals were acquired, amplified, filtered, processed and further amplified to scale up to ADC range. In total, there were 8 processed analog signals. A SPI 12 bit ADC with 8 channel inputs was used to convert these into Digital data to be fed to a MCU.

The signal conditioning and amplifying used 14 analog ics, around 100 resistors, 40 capacitors, protective diodes, ferrite beads apart from a DC DC converter to generate positive and negative voltages. The PCB size was approximately 140 mm by 120 mm.

Though this worked, we wanted something better, faster and smaller. When an AFE from TI was introduced, we jumped at it. It was a single SMD IC occupying very small space and replaced all those components including the ADC excluding the protective diodes and ferrite beads. Added to this, we got 24 bit resolution.

It seems to me that the "embedded" specs work best when the integrated component has a single function and there's little or no deviation allowed from the target function. Ideally, if the component has enough flexibility to be used in other applications, then the developer needs to assume that someone will want to use it in those other applications. It's best if the specs are comprehensive enough to allow the designer to predict the performance in "non-target" applications.

And yes, there have been cases where a "hidden" spec was added to a data sheet after an IC had been on the market for a while; in those cases it would have been easier for everyone if the specs for the individual blocks had been listed on the data sheet from the beginning.

Every case is different, but in general IC developers do think about non-target applications where their products might be used, and have to make a decision about whether to publish the detailed specifications that would help in using the product in those applications. Cost is often a factor, as the additional specs may require more testing. So a chip that's aimed at a very low-cost, high-volume application may not meet its cost goals if the testing is done to guarantee all of those "internal" specs that would make it ideal for other designs.

Hi Kerry--there has been some other discussion here about the specs associated with integrated parts. Your point about "embedded" specs is good-if the input and output specs meet your needs, you can treat some of the block as a black box. Some conerns have been expressed that there are subtleties that can get buried in the integrated part. Have you seen any issues were behavior related to either (a) behavior not specified by the integrated part data sheet (i.e. off spec use) or (b) lack of specs at the higher level casued designers headaches further downstream (like in full board test?)?

Hi Kerry I'd love to see the details of any real world examples where the design using integrated analog, would be great to learn about the decisions and tradeoffs that the designer had to make. Would be a great roadmap for engineers as they evaluate this solution in their own designs.

I have to agree to, "the right chip not only integrates circuit blocks, it also integrates time". All the customer wants is a single chip solution so that he may not invest much time into debugging the system, all he has to do is check if that chip is not working or meeting the specifications and report it back to the vendor. The component list and BOM of the customer reduces and the system debug time.

But as the integration increases, one has to be critical about the die size and package, testing time/cost, EM, on-chip temperature variations and reliability...